Electromagnetic wave modulating devices



' Sept. 19, 1961 A. L. MORRIS 3,001,151

ELECTROMAGNETIC WAVE MODULATING DEVICES Filed March 12, 1958 COOLING a CIRCuLATION TANK 2 5 8 DIELECTRIC T 2 l BULKHEAD 9 T FERRITE I COOLING a CIRCULATION RADIATOR 4 IJ 4A I I MW 25 \E I1 I DIELECTRIC 3 B BULKHEAD FIG. 2

I $8M ln ntor WIBJQI Attorney 3,001,151 ELECTROMAGNETIC WAVE MODULATING DEVICES Arthur Langley Morris Malvern, England, assignor to National Research Development Corporation, London,

England, a British corporation Filed Mar. 12, 1953, Ser. No. 720,892 Claims priority, application Great Britain Mar. 15, 1957 1h (llaims. (Cl. 332-51) The invention relates to electromagnetic wave modulating devices of the kind in which gyro-magnetic material is carried in a length of waveguide and modulation of a wave propagated in the waveguide is obtained by varying that component of the magnetic flux in the material which lies in the direction of propagation along the waveguide.

The operation of a device of this kind can be explained generally in terms of the Faraday effect. A material of the ferrite type is often employed as the gyro-magnetic material, hence the name ferrite modulator which is generally used. Ferrite materials can be found which have low loss at microwave frequencies and accordingly modu lating devices using them can be made to operate at these frequencies.

In the achievement of practical devices however there are ditflculties, which mainly concern the means adopted for varying the magnetic flux in a modulating device in accordance with a given modulating signal.

One method of applying a modulating signal to vary the flux is to provide a solenoid wound around the waveguide; in that case eddy currents are induced in the waveguide walls by the modulating field and set a limit to the modulating frequency. In an alternative method in which the solenoid is wound inside the waveguide there is difiiculty owing to the presence in the wave propagating path of the Winding of the solenoid.

It is an object of the invention therefore to provide an improved modulating device of the kind referred to.

According to the invention in -a modulating device of the kind referred to, the Wall of the waveguide defines a current path for a modulating signal extending in onehanded direction around the axis of propagation of the waveguide and along the length thereof.

In order to make the invention clearer two examples of ferrite modulators in which the invention is used will now be described with reference to the accompanying drawings, in which:

FIG. 1 shows a partly sectioned view of one form of ferrite modulator, and I FIG. 2 shows a further form of ferrite modulator.

In FIG. 1 a thin conducting circular tube 1 carries a narrow helical slot 2 which progresses along the tube 1. The tube '1 is supported by a body 3 of plastic material which is conveniently an epoxy resin such as that known under the registered trademark Araldite. The body 3 is transparent and hence in the drawing it has been possible to indicate the tube 1 as visible through the body 3. An end flange 4 is provided at the left-hand end of the tube 1 as shown in the drawing and is in electrically conducting engagement with it except where a short longitudinal slot 5 continues the helical slot 2 from the end of its helical path to the end of the tube 1. At the other end of the tube It a second end flange 6 is similarly fixed except that it does not make conducting contact with the end of the tube 1. A terminal 7 is mounted on the plastic body 3 and connected via a lead 8 on the body 3 to the end of the spiral Which'the tube 1 now forms owing to the slot 2. The end of the slot 2 continues to the end of the tube 1 by means of a further short longitudinal slot; in the drawing this is obscured by the flange '6. V l

hddLlSl Patented Sept. 19, 1961 A red 9 of ferrite material, for example a magnesiummanganese ferrite is located coaxially of the tube 1 by means of an annular foam spacer 10.

In operation the device of FIG. 1 is secured into a desired waveguide transmission system by the flanges 4 and 6 at its ends; suitable circular to rectangular waveguide transformers are provided if necessary and an earthed source of high frequency modulating signal is connected to the terminal '7.

A modulating signal current flows along the spiral of the tube 1 from the terminal 7 to the end flange 4; the end flange 4 is effectively earthed to the rest of the waveguide system; but the flange 6, although connected to the waveguide system, is insulated from the tube 1 and does not form part of the modulating signal current circuit. The modulating signal current thus flows helically about the ferrite rod; and the flux in the ferrite rod 9 varies according to the modulating signal current flowing round it.

The result is that the propagation of a wave in the circular tube 1 is controlled in accordance with the modul The copper is not lating signal applied to the terminal 7 and the wave is effectively modulated.

In the construction of the device of FIG. 1 the slot 2 is made small to minimise radiation outwards from inside the tube 1. The thickness of the tube 1 is made small; it should be thick enough to allow waves to propagate along the Waveguide system of which it forms part; on the other hand it should be small to reduce eddycurrent losses in it.

The number of turns of the coil which is effectively formed by the slot 2 in the tube 1 is chosen from considerations of the modulation signal frequency and the frequency of the wave propagated down the tube 1, bearing in mind the magnitude of the modulating current which flows during operation of the device.

In a typical example the circular tube 1 was 0.9 in. in diameter; the overall length of the tube section was 5.5 in. and con1prisedl4 turns pitched at 3 turns per inch. The diameter of the magnesium-manganese ferrite rod was 0.25 in. A wave of frequency 9375 mc./s. propagated along the tube 1 was modulated when a current of 1 amp. at 7 mc./s. was fed to the tube ll at the terminal 7. The insertion loss was 0.5 db. t

The preferred method of manufacture for the tube 1 is that due to E. B. Cowley & G. W. Fynn and is described in copending patent application No. 8,593/57. Briefly, the method involves a stainless steel mandrel of a length and outside diameter corresponding to the length and inside diameter of the tube which provides the helix. A helical track and longitudinal lead tracks at each end are cut in the mandrel and filled with an insulating plastic filler, e.g. an epoxy resin such as Araldite, to bring them flush with the surface again. A layer of copper is then deposited on the mandrel in an elcctroforming bath. deposited over the plastic-filled track and the required tube is thus formed having a helical slot along its length.

After elcctro-forming a coating of an epoxy resin is applied to the tube, whilst still on the mandrel; the tube is then removed from the mandrel and the end flanges fitted.

The cross-section of the slot due to the presence of the; track narrows as the thickness of the copper is increased; although it can be made narrow in any event, it gets still narrower as electro-forming proceeds. Providing, of course, care is taken so that the slot does not bridge over, the slot gap can be made so narrow at the surface of the copper that, in operation of the modulator, there is little leakage of microwave radiation through the slot.

alternative method proposes the use of printed Wiring techniques to provide the tube 1.

A further form of modulator is shown in FIG. 2 where two circular tubes 1A, 1B each carry a helical slot 2A, 23 respectively; the helical slots 2A, 2B are arranged so that they are wound about the axis of the tubes 1A, 1B in opposite-handed directions. The tubes 1A, 1B abut together at a common edge 12. but are insulated there from each other. The two tubes 1A, 1B are connected in a waveguide system at their end flanges 4A, 43 again respectively.

The flanges 4A, 4B are electrically connected to their respective tubes 1A, 1B and provide an earthed connection because they are connected to the tubes 11 of the waveguide system.

As in the modulator of FIG. 1 the tubes 1A, 1B are supported by a body 3 of an epoxy resin. The tubes EA, 1B similarly enclose an annular foam spacer and a rod of ferrite material, both of which extend between the flanges 4A, 4B. A terminal '7 is held in the resin body 3 and is connected at the abutment edge 12 to the tubes 1A, 1B. 7

In operation an earthed source of high frequency modulating signal M is connected at the terminal 7 and the tubes ill of the waveguide system feed microwave energy through the tubes 1A, 18; a circuit is thus completed from the earthed source of modulating signal via the connection to the terminal 7 and the contrary-wound helices formed in the tubes 1A, 113 to the earthed flanges 4A, 4B. The result is that the microwave energy in the waveguide system is modulated by the signal from the source M. This construction has the advantage that the flanges are at earth potential and that there is no completely annular break in the waveguide system; only one feed, the terminal 7, is required for the two tubes 1A, 1B and no flanges are required where they abut. This simplifies the electrical arrangement of the modulator. Moreover, in the manufacture of the modulator it is proposed to make the tubes 1A, 1B in one operation, that is, on a single mandrel by electroforming.

Where it is desirable to reduce radiation through the helical slot of a modulator, or to be able to reduce the thickness of the ferrite rod 9 to reduce heating in it a tube of 'high dielectric constant material can be substituted for the annular foam spacer 10 f FIG. -1. Suitable materials are polystyrene and 'polytetrafluorethylene ('P.T.F.E. boron nitride would also be suitable and would be of additional advantage owing to greater thermal conductivity. It is' proposed to extend this arrangement toprovide for increased cooling by using a fluid dielectric instead of the dielectric tube; dielectric bu1kheads would be provided across the waveguide to contain the fluid dielectric. For high duty modulators it may be desirable to provide some coolingif at all possible and it is proposed by the use of a silicone oil, or possibly carbon tetrachloride, as a liquid dielectric to provide a means of cooling the modulator. Suitable circulation and cooling arrangements could also be provided for the fluid. For instance a cooling radiator would be connected to the waveguide by fluid-carrying connections through small holes in the wall of the waveguide.

WhatIclaim is:

l. A modulating device of the kind-referredto, comprising a waveguide and a mass of gyromagnetic mate,-

rial located within the waveguide wherein the wall of the waveguide defines a current path for a modulating signal extending in one-handed direction around the axis of propagation of the waveguide and along the length thereof.

'2. A modulating device of the kind referred to, comprising a waveguide and a mass of gyromagnetic material located within the waveguide wherein the wall of the waveguide defines two current paths for a modulating signal each extending from a common point in opposite- 4 handed directions around the waveguide and in opposite directions along the length thereof.

3. A modulating device as claimed in claim 2, wherein the length of waveguide is terminated by flanges connected electrically with the waveguide wall and constituting ends of the current paths remote from the common point, and a connection is made at the common point, whereby, in operation with the modulator connected at its flanges into an associated waveguide system, the current paths are fed in parallel from a modulating current source connected between the common earth connection formed by the waveguide system and the connection at the common point.

4. A modulating device as claimed in claim 2, wherein the current paths are of helical form in the wall of a circular waveguide, said helical paths being delineated by helical slots in said waveguide wall, said slots being wider'on the inside of the waveguide wall than on the outside.

5. A modulating device as claimed in claim 1, wherein the current path is delineated by aslot in the waveguide wall extending end-to-end round the axis of the wav guide, the slot being wider on the inside of the waveguide than on the outside.

6. A modulating device as claimed in claim 5, wherein a layer of insulating material is applied to the outside of the slotted waveguide for its support.

7. A modulating device as claimed in claim 6 wherein the gyromagnetic material is in the form of a rod and carries a sleeve of dielectric material.

8. A modulating device as claimed in claim 5 wherein the gyromagnetic material is in the form of a rod and is surrounded by a fluid dielectric material, and means are provided'for cooling and circulating the fluid material.

9. An electromagnetic wave modulating device in which a wave propagated in a waveguide is modulated by vsrymaterial in the waveguide which lies in the direction of propagation along the waveguide comprising, a waveguide, a mass of gyromagnetie material located within the waveguide, the'wall of the Waveguide defining a current path for a modulatin'g signal extending in one-handed direction around the axis of'pjropagation of thewaveguide and longitudinally of the waveguide, and connections to the waveguideat each end of the current path defined thereby for connection to a modulating signal source.

10. An electromagnetic wave modulating device in which a wave propagated in a waveguide is modulated by varying that component of magnetic flux in -a gyromagnetic material in the waveguide which lies in the direction of propagation along the waveguide comprising, a waveguide, a mass of gyromagnetic material located within'the' waveguide, the wall of the waveguide defining two current paths for'a modulating signal each extending from a common 'point in opposite-handed directions around the waveguide and in opposite directions alo'ng'the length thereof.

References Cited in thefile of this patent UNITED "STA S PATENTS P b- 7 Nation l Convention Record of the IR-E, Pait I, pp. 227-234,

mg that component of magnetic flux in a gyroma'gne'tic 

